NASA has announced that is has successfully tested the first deep space RS-25 rocket engine for 500 seconds–-a critical milestone that means the rocket is almost ready to carry human occupants into deep space. The engine is one of four that will make up the Space Launch System (SLS) that will enable a new era of space exploration, including manned trips to asteroids and Mars.

“What a great moment for NASA and Stennis,” Rick Gilbrech, director of NASA’s Stennis Space Center in Bay St. Louis, Mississippi, said in a statement. “We have exciting days ahead with a return to deep space and a journey to Mars, and this test is a very big step in that direction.”

Photo:NASA/SSC

You might assume that the RS-25 rocket engines used in the SLS are new technology, but they are actually the “workhorse engines” that have powered 135 space shuttle missions between 1981 and 2011. When the four engines are combined into the SLS, they will have a combined 2 million pounds of thrust.

“Not only does this test mark an important step toward proving our existing design for SLS’s first flight,” Steve Wofford, engines manager at NASA’s Marshall Space Flight Center in Huntsville, Alabama, said in a statement. “But it’s also a great feeling that this engine that has carried so many astronauts into space before is being prepared to take astronauts to space once again on SLS’s first crewed flight.”

NASA says the SLS’s first flight will happen no later than November 2018.